A flyback switching power converter is typically used to charge a mobile device as the converter's transformer provides safe isolation from AC household current. It is conventional for the switching power converter to couple to the device being charged through a standard interface such as a Universal Serial Bus (USB) interface. The USB interface includes a differential pair of signals (D+ and D−) for signaling and also provides power and ground. With regard to the delivery of power, a USB cable can only provide a certain amount of current. For example, the USB 2.0 standard allows for a maximum output current of 500 mA whereas the USB 3.0 standard allows a maximum output current of 900 mA. Traditionally, the delivery of power through a USB cable occurs using an output voltage of five volts. But modern mobile device batteries typically have a storage capacity of several thousand milliamps. The charging of such batteries, even at the increased output currents allowed in the USB 3.0 standard, would thus be delayed if the power is delivered using a five volt output voltage. This charging delay is exacerbated since the switching power supply, the USB cable, and the receiving device all present a resistance to the output current.
To enable a rapid charge mode in light of the output current limitations and associated losses from device resistances, it is conventional to use markedly higher output voltages over the USB cable. For example, rather than use the default USB output voltage of 5 V, rapid charging modes have been developed that use 9V, 12V, or even 19V. The increased voltages allow the switching power supply to deliver more power over the USB cable without exceeding the maximum output current limitations. However, many legacy devices can only accomodate the standard 5V from a USB cable. A rapid-charge switching power supply will thus engage in an enumeration process with the device being charged to determine if the higher output voltages are supported. This enumeration may occur over the differential D+ and D− pins in the USB interface. Through the enumeration, the switching power converter and the enumerated device may change the USB output voltage to an increased level that is supported by the enumerated device. The result is considerably reduced charging time, which leads to greater user satisfaction.
Although rapid charging modes are thus advantageous, problems have arisen with regard to their implementation. For example, the USB cable interface may get dirty such that a dust particle or other slightly conductive object couples between the VCC pin (the pin delivering the output voltage) and one of the differential signaling pins D+ and D−. Alternatively, the USB cable itself may become frayed from twisting by a user such that a slightly conductive path exists between the VCC wire and one of the wires for the D+ and D− signals. The result is a “soft short” between VCC and one of the differential data signals in the USB cable. It is denoted as a soft short in that the impedance for the coupling between the corresponding pins (or wires) is relatively high compared to a true short circuit. With regard to true short circuits, it is conventional for a switching power converter driving a USB cable to include an over-current protection circuit that will shut down the charging through the USB cable if a short circuit is detected. In this fashion, the maximum output current levels for the USB interface are not exceeded. But a soft short will not result in such a large increase in current. A conventional switching power converter with overcurrent protection will thus not respond to a soft short in that the increase in output current is negligible or minor such that it does not trigger an over-current state.
If the output voltage (VCC) is 5V such as was traditional for a USB interface, a soft short does not result in a dangerously elevated voltage level on the differential signaling pins as, by definition, a soft short involves a relatively high-impedance path. But as the output voltage is increased to support rapid charging, the differential signaling pin voltage may be driven to an unsafe level. For example, the receiving circuitry for the differential signaling in the device being charged may be harmed by the elevated differential signaling voltages.
Accordingly, there is a need in the art for improved power converters that protect against soft shorts over data interfaces.